1. Field of the Invention
The invention relates generally to thin film devices and, more particularly, to thin film bipolar transistors.
2. Discussion of the Related Art
The active semiconductor layers of some bipolar transistors are vertically stacked normal to the surface of the substrate on which the transistor rests. The vertical stacking produces a nontrivial cross-sectional profile for the transistor.
Such vertical stacking is illustrated by the triple mesa structure 10 shown in FIG. 1. The triple mesa structure 10 includes a series of semiconductor layers that includes emitter, base, and collector layers 11-13 and a heavily doped subcollector layer 14. The series forms a vertical stack on top surface 15 of a semiconductor substrate 16. The triple mesa structure 10 also includes electrical contacts 17, 18, and 19 for the emitter, base, and subcollector, respectively. In an integrated circuit (IC), a protective dielectric layer 9 covers the triple mesa structure 10.
In the triple mesa structure 10, vertically stacking of semiconductor layers can lead to undesirable properties. First, the structure often provides poor dissipation of heat during transistor operation. Second, the structure often reduces adhesion of the protective dielectric layer 9 to the underlying substrate 16. It is desirable to have structures for bipolar transistors with improved properties.